The dessert stomach appears in childhood, and typically gets smaller as one ages, though this is not always the case.
CHILD: *Groan* I can't! I'm so full, I think I'll barf if I eat another bite.
MOM: Well, I guess you're too full to eat dessert.
CHILD: I'm not too full! My dessert stomach is empty!
(child proceeds to eat entire dessert portion)